The present invention relates to a process for the automatically controllable reduction of the nitrite content of nitrite-containing aqueous solutions, more particularly wastewaters and pickling acids, to values below 1 mg NO.sub.2.sup.- /l by treatment of the solutions at a pH value below 3.5 with hydrogen peroxide of which the addition is controlled by the redox potential. The process according to the present invention also encompasses the treatment of nitrite-containing aqueous solutions which, in addition to nitrite, contain other compounds oxidizable by hydrogen peroxide.
An automatically controllable process for detoxifying wastewaters containing nitrite ions, in which the nitrite ions are oxidized with hydrogen peroxide to nitrate ions in the mildly acidic pH range, is known from DE-PS 28 52 475 (U.S. Pat. No. 4,280,914). In this process, the reduction in the pH value during oxidation is used to control the input of H.sub.2 O.sub.2. This process is suitable for detoxification by batch methods, but not for the treatment of large wastewater streams with fluctuating contents of various ingredients oxidizable with H.sub.2 O.sub.2 by continuous methods.
EP-A-0 267 166 describes a process for the reduction of NO.sub.X gas from a nitric acid solution containing "dissolved NO.sub.X " by treatment of the solution with hydrogen peroxide, the quantity of hydrogen peroxide added being controlled depending upon the redox potential. FIG. 1 of that document shows the redox potential curve as a function of the "dissolved NO.sub.X " content which comprises nitrous acid, N.sub.2 O.sub.3, NO and NO.sub.2. It can be seen that the redox potential increases with decreasing content of dissolved NO.sub.X, reaching a maximum of +855 mV on complete oxidation, i.e. at an NO.sub.X content of zero. When more H.sub.2 O.sub.2 is added, the redox potential of the solution containing excess H.sub.2 O.sub.2 falls to +650 mV.
Contrary to the teaching of EP-A 0 267 166, it has now been found that nitrite-containing wastewater treated with hydrogen peroxide by the process according to that document is not free from nitrous acid at the maximum redox potential, which is in the order of +850.+-.30 mV depending on the acid concentration, so that nitrite remains in the waste-water after neutralization and leads to environmental pollution. This process fails completely unless sufficient iron (II) ions are present in addition to nitrite ions in the solution to be treated. In this case, the initial potential is itself +850 mV so that, under the known control regime, no H.sub.2 O.sub.2 at all would be added and, hence, no detoxification would occur.